Add support for silently filtered words

[SlimShadyIAm]

Closes #39

Description

We want to be able to detect when certain keywords are mentioned and notify moderators without removing the message.

Changes

• Extend FilterWord model with new silent_filter property which defaults to false
• New command /silent_filter to toggle silent filter mode on already filtered words
• /filter list shows when a word is silently filtered
• Ignore silently filtered words in commands that typically stop executing/return when a filtered word is triggered (i.e when you search a package in Canister with a filtered word as the query)
• Generate report for silently filtered words, but don't delete the word, ratelimit the user or send them a message privately

[BBaoVanC]

I am not familiar enough with the codebase to do a meaningful review, so that is what I have found